The Earliest Candidate Viking Landing Sites (1970)

Image: NASA

The U.S. Congress approved new-start funding for Project Viking, the successor to ill-starred Project Voyager, in October 1968. In the Viking mission plan NASA presented to Congress, two Viking missions would leave Earth in 1973. Each would include an orbiter and a lander. The former would be based on the Mariner family of flyby probes, five of which had flown as of late 1968. The Pasadena-based Jet Propulsion Laboratory would build the Viking orbiters, just as it had built the flyby Mariners.

The Viking lander design was, by contrast, far from settled. This was troublesome in part because it had the potential to impact the orbiter design. Two design choices drove the sometimes heated debate: at which point in the mission the lander should separate from the orbiter and how the lander should touch down on Mars’s surface.

The lander might separate from the orbiter as it approached the planet and enter Mars’s atmosphere directly with no stop in Mars orbit. The orbiter, relieved of the lander’s mass, would need to carry only enough rocket propellants to slow itself so that Mars’s gravity could capture it into orbit.

Alternately, the lander might separate after the orbiter had captured into Mars orbit. In that case, the orbiter would need to carry enough propellants to brake both itself and the lander. The lander would need deorbit propulsion so that it could slow and drop into the martian atmosphere.

At one end of the spectrum of possible lander designs was a soft-lander, which might return scientific data from Mars’s surface for months. Its longevity made it the option most favored by scientists. At the other end of the spectrum was an impact capsule, which might return Mars atmosphere data and surface images for only minutes as it plummeted toward destruction. Somewhere in between the two extremes was a rough-lander, which might descend on a parachute and return data from Mars’s surface for a few hours after touchdown.

On 5 December 1968, lame-duck President Lyndon B. Johnson’s Bureau of the Budget agreed with NASA officials that the Viking lander should separate from the orbiter in Mars orbit and soft-land on Mars. Though the selection was welcomed by scientists, it was the most complex, massive, and costly Viking lander design option.

Mockup of early Viking lander design. Image: NASA

Serious consideration of candidate Viking landing sites commenced almost as soon as NASA settled on a mission design. The scientists and engineers taking on the site selection task had very little data with which to work, however. In fact, they had close-up images of only 1% of Mars’s surface. The Mariner IV spacecraft had captured 21 grainy black-and-white images of the planet as it flew by on 14-15 July 1965. They also had topography data collected using Earth-based radar in 1967, when Mars passed within 90 million kilometers of Earth. Besides these data, they had only photos, drawings, and conjecture from more than a century of Earth-based telescopic observation.

Landing site selectors could, however, look forward to data from Mariner 6 and Mariner 7, scheduled for launch in February-March 1969. The twin spacecraft would fly past Mars in late July-early August 1969. They also anticipated renewed attempts to chart Mars’s topography using radar during May-June 1969, when the planet would pass within 72 million kilometers of Earth.

Even before planners received the 1969 data, however, preliminary Viking landing site discussions had become useful. For one thing, they helped engineers and scientists to develop imaging system requirements for the Mariner 8 and Mariner 9 spacecraft, which were scheduled to orbit Mars together and image its surface from pole to pole beginning in late 1971.

Mariner 6 and Mariner 7 returned a total of 201 new close-up Mars images. In late 1969, the Army Map Service created for NASA a map that featured swatches of Mars’s surface the twin flyby spacecraft had imaged up close. This became the base map for the first map of preliminary candidate Viking landing sites (image at top of post). The landing site map bears no date, but almost certainly was drafted for the third meeting of the Viking Landing Site Working Group (2-3 December 1970).

Detail of the “Viking Zone of Interest” showing preliminary landing site candidates A-1, B-1, and B-2, elevated regions including Syrtis Major, and part of the area Mariner 7 imaged (left). Image: NASA

The map uses the romantic-sounding classical names that Earth-based telescopic observers gave to martian light and dark features over more than a century of observations. Many of the names that appear on the map are now obsolete or are used in modified form.

Dashed lines at 30° north and 30° south mark the bounds of the equator-centered “Viking Zone of Interest.” Planners assumed that the Viking orbiter/lander combinations would capture into near-equatorial orbits, limiting potential Viking landing sites to relatively low latitudes.

On the map, the regions Earth-based radar revealed to be of high elevation are outlined using red dots. Other lines of red dots point up-slope toward the centers of the raised regions. Landing site planners treated the high-elevation areas as no-go zones because high elevation equates with low atmospheric pressure. Viking was expected to descend at least part of the way to the surface on a parachute; if air pressure were too low and elevation too high, the parachute would not become effective before the lander reached the ground. Although details are different, a similar engineering constraint governs Mars landing site selection today.

Solid red ellipses mark Mission A (Viking 1) candidate sites, all of which are located north of Mars’s equator. Site A-1, the primary ellipse, is labeled Thoth-Nepenthes, but spans Isidis Regio, a light-colored area close to Syrtis Major, the darkest martian surface feature. Because of its dark hue, which some believed suggested the presence of plant life, Syrtis Major was of keen interest to scientists; unfortunately, radar revealed it to be of high elevation. Isidis Regio corresponds to Isidis Planitia on modern Mars maps. Low-lying Isidis was, incidentally, the target of the ill-fated British Beagle II lander, which vanished without a trace on Christmas Day 2003.

Site selectors centered the first Mission A back-up landing ellipse, designated A-2, at 30° north in Niliacus Lacus, a diffuse dark feature on the southern edge of Mare Acidalium. It was the most northerly of the Mission A ellipses. The second back-up ellipse, designated A-3, they placed in light-colored Amazonis between the high-elevation Tharsis and Elysium regions.

Mission B (Viking 2) primary and back-up landing zones, marked on the map by solid green ellipses, are all south of the martian equator. All occur in areas at least partially imaged by Mariner 7. The B-1 landing ellipse, centered just inside the circular, light-colored Hellas region at 30° south, is the most southerly of the Mission B candidates. Oddly enough, though selected as the primary Mission B target, it includes the least amount of Mariner 7 image coverage of the three B landing ellipses.

B-2, on the other hand, is entirely located in Mariner 7-imaged terrain, near the martian central meridian in light-colored Pandorae Fretum. On modern Mars maps the region corresponds to far northern Noachis Terra, a heavily cratered region that now lends its name to the oldest officially named era of martian geologic history. The Noachian ended about 3.7 billion years ago.

Detail of the “Viking Zone of Interest” showing preliminary landing site candidates A-2, A-3, and B-3, the elevated Tharsis region, and part of the area Mariner 7 imaged (right). Image: NASA

Site B-3, in Aurorae Sinus, demonstrated the limitations of Mariner flyby images. About half of the B-3 ellipse lay within the zone of Mariner 7 image coverage. Mariner 7 images of the region include features which scientists grouped together under the catch-all label “chaotic terrain.” Many of these features are in fact scattered parts of the Valles Marineris canyon system, a rift valley that extends along the martian equator for 4000 kilometers. Though they had viewed parts of it, scientists did not suspect that the mighty canyon existed until Mariner 9 imaged it beginning in late 1971-early 1972.

On 7 December 1970, Viking Project manager James Martin directed Viking prime contractor Martin Marietta to assume for spacecraft design purposes that the Viking 1 lander would set down in Thoth-Nepenthes and Viking 2 would land in Hellas. These thus became the first “official” primary Viking landing sites. There would be many others.

Mariner 9 flew to Mars orbit alone after Mariner 8 crashed in the Atlantic, the victim of launch vehicle failure. Careful mission contingency planning meant that it was able to achieve the exploration objectives of both spacecraft. It returned to Earth more than 7000 images over 11 months. On the basis of its images, scientists and engineers selected new Viking candidate landing sites. The primary Viking 1 site became Chryse Planitia, a region of sinuous and braided channels apparently carved by floods. Viking 2 was targeted for Cydonia. The region, long considered by telescopic observers to be of special interest for its perceived unusual coloration, was in the “transition zone” between Mars’s old cratered southern highlands and young smooth northern lowlands.

Landing site planners had plenty of time to carefully pick Viking sites based on the Mariner 9 images, for funding shortfalls delayed the Viking launches from 1973 to 1975. Nevertheless, when Viking 1 at last arrived in Mars orbit on 19 June 1976, its cameras, which were improved over those of Mariner 9, returned images that showed that the primary and back-up Viking 1 landing sites were too rough to permit safe landings. NASA postponed Viking 1’s planned 4 July 1976 landing while exasperated landing site planners began a hurried search for a new site. On 20 July, Viking 1 separated from its orbiter, fired its deorbit rocket motors, descended through the atmosphere, and touched down on a rocky plain a few hundred kilometers north of its original primary site. Viking 1 was the first successful Mars lander.

The primary and back-up landing sites for Viking 2 were also found to be too rough, so site planners redirected it to Utopia Planitia, a nearly featureless plain a third of the way around Mars from its original planned primary site. Viking 2 landed safely on 3 September 1976.

Viking 1 view of its landing site in Chryse Planitia. Trenches near the lander were excavated using its arm-mounted scoop tool. Image: NASA